Optimization of a process for denitrogenation of a natural gas stream

ABSTRACT

A process for the separation of the components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, including introducing the gas mixture to be treated into a first distillation column thereby producing, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of the column, another gas stream; and introducing the other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of the second column, thereby producing, at the top of this column, a second gas stream rich in methane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International ApplicationPCT/FR2016/052557, filed Oct. 5, 2016, which claims priority to FrenchPatent Application 1560529, filed Nov. 3, 2015, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the separation of thecomponents of a gas mixture containing methane, nitrogen and heavierhydrocarbons than methane.

The present invention thus applies to processes for the denitrogenationof natural gas, with or without recovery of helium.

Natural gas is desirable for use as fuel intended to be used for heatingbuildings, in order to provide heat for industrial processes, for theproduction of electricity, for use as starting material for varioussynthetic processes for producing olefins, polymers and the like.

Natural gas occurs in numerous fields which are at a distance from theusers of natural gas. Natural gas is typically composed of methane (C₁),ethane (C₂) and heavier compounds, such as hydrocarbons having at leastthree carbon atoms, such as propane, butane, and the like (C₃+).

It can often be advantageous to separate the C₂ and C₃+ compounds fromthe natural gas in order to market them as separate coproducts. This isbecause their value is generally greater than natural gas itself as theycan be used directly for chemical processes (manufacture of ethylenefrom ethane, for example), as fuels (C₃/C₄ is a conventional fuel knownas LPG) or for many other applications.

Another component often present in natural gas is nitrogen. The presenceof nitrogen in natural gas can cause difficulties in observing thespecifications for natural gas (typically minimum net calorific value tobe observed). This is all the more true when the heavier hydrocarbonsthan methane (C₂ and C₃+ compounds) are removed as these have a highernet calorific value than methane; by removing them, the net calorificvalue is thus reduced, and it may then be necessary to increase it bythe separation of nitrogen.

Consequently, a considerable effort has been devoted to the developmentof means for removing the nitrogen present in natural gas.

The natural gas deposits exploited increasingly contain nitrogen. Thisis explained in particular by the exhaustion and increased scarcity offields which are sufficiently rich for no enriching treatment to benecessary before the marketing of the gas.

Frequently, these natural gas sources also contain helium. The lattercan be made economic use of by carrying out a preconcentrating, beforefinal treatment and liquefaction.

Unconventional resources, such as shale gases, also have the sameproblem: in order to make them marketable, it may prove to be necessaryto increase their calorific value by means of a treatment which consistsin denitrogenating the gas.

The most widely used method for separating nitrogen and heavierhydrocarbons than methane is “cryogenic separation”. A cryogenic processfor the separation of nitrogen, more specifically a process employing adouble column, is described in the patent application U.S. Pat. No.4,778,498. Natural gas denitrogenation units generally treat gases whichoriginate directly from wells at a high pressure. After denitrogenation,the treated gas has to be sent back to the network, often at a pressureclose to its entry pressure.

During the exploitation of natural gas deposits, numerous steps may beprovided. A relatively conventional step after the drying and thewithdrawal of the impurities is the separation of the liquids associatedwith the natural gas (NGLs). This step can have many advantages butoften it is a matter of making economic use of various “heavy”hydrocarbon products containing at least two carbon atoms (C₂, C₃, andthe like, compounds) which are generally sold much more expensive thanthe natural gas product. If the natural gas contains nitrogen, there isa risk of re-encountering a natural gas having an excessively lowcalorific value because of the low resulting content of C₂, C₃, and thelike, compounds. It is thus typical to then have to separate thenitrogen from this gas in order to make it marketable.

A conventional solution is to treat the two problems independently.

A first unit carries out the separation of the NGLs (subsequently knownas NGL unit), while a second unit separates the nitrogen from thenatural gas (subsequently known as NRU). This solution exhibits theadvantage of operational flexibility. For example, if the NRU unitcomprises a refrigeration cycle, the associated devices have a limitedreliability, and a failure of a cycle compressor will result in theshutdown of the NRU but without resulting in the shutdown of the NGL.

Unfortunately, this shutdown cannot be lengthy in duration since it willthen be necessary to send the production to the flare stack (because ofits excessively low calorific value). In addition, this scheme islimited in terms of effectiveness as all the gas is cooled and thenreheated in the NGL unit and then cooled and reheated in the NRU.

During a treatment in an NGL separation unit, a significant fraction(typically more than 10%) of the feed gas is condensed. During thiscondensation, methane is condensed with the heavier hydrocarbons (C₂+and/or C₃+ compounds). It is then typically necessary to use a columnknown as a demethanizer in order to reboil the methane and not to losemethane in the C₂+ and/or C₃+ products. If nitrogen is present, thelatter will, on the other hand, be only very slightly condensed and willbe re-encountered predominantly in the gas phase introduced into thedemethanization column.

SUMMARY

The inventors of the present invention have thus developed a solutionwhich makes it possible to solve the problem raised above whileoptimizing the energy costs, such as, for example, those related to theelectrical consumption during the implementation of such processes.

A subject matter of the present invention is a process for theseparation of the components of a gas mixture to be treated comprisingmethane, nitrogen and at least one hydrocarbon having at least twocarbon atoms, or a mixture of these hydrocarbons, comprising thefollowing steps:

a) introduction of the gas mixture to be treated into a firstdistillation column in order to create, at the column top, a first gasstream enriched in methane and, at a level lower than that of the top ofsaid column, another gas stream;

b) introduction of said other gas stream resulting from step a) into asecond distillation column, at a level lower than that of that of thetop of said second column, in order to create, at the top of thiscolumn, a second gas stream rich in methane,

characterized in that the nitrogen content of said second gas stream isat least 1.5 times lower than the nitrogen content of the first gasstream and in that from 5 mol % to 30 mol % of the methane initiallypresent in the gas mixture to be treated is comprised in the second gasstream.

More particularly, a subject matter of the present invention relates to:

A process as defined above, characterized in that it comprises theadditional stage:

c) introduction of said first gas stream enriched in methane resultingfrom step a) into a denitrogenation unit in order to separate thenitrogen from the other components of this gas stream.

A process as defined above, characterized in that the second gas streamresulting from step b) is not treated by the denitrogenation unit.

A process as defined above, characterized in that, prior to step a), itcomprises the following steps:

-   -   at least partial condensation of said gas mixture to be treated        in order to obtain a two-phase mixture;    -   injection of the liquid phase from said two-phase mixture into        said first demethanization column at a first injection stage;    -   injection of the vapor phase from said two-phase mixture into        said first demethanization column at an injection stage        different from said first stage.

A process as defined above, characterized in that the gas stream,extracted from the first distillation column in step a), comprises atmost half of the amount of hydrocarbons having more than two carbonatoms present in the feed gas.

A process as defined above, characterized in that said gas mixture to betreated comprises at least 70 mol % of methane, at least 4 mol % ofnitrogen and 2 mol % of hydrocarbons having at least two carbon atoms.

A process as defined above, characterized in that said gas streamresulting from step b) is extracted directly from said seconddistillation column at a pressure of greater than 20 bara and comprises95 mol % of methane.

A process as defined above, characterized in that a portion of thesecond gas stream rich in methane and depleted in nitrogen resultingfrom step b) at the outlet of the top of the second distillation columnis compressed and then condensed in order to be introduced, for oneportion, into the upper part of the first distillation column and, forthe other portion, into the upper part of the second distillation columnin order to carry out the reflux of said distillation columns.

Thus, the process which is a subject matter of the present inventionmakes it possible to take advantage of the fact that only the gas feedof the methanization column substantially contains nitrogen.

This is because the solution provided, in comparison with the knownprocesses of the state of the art, is that of splitting in two thedemethanization column conventionally used, one (that is to say, thefirst distillation column) producing a natural gas product poor in C₂+and rich in nitrogen and the other (that is to say, the seconddistillation column) containing a natural gas product poor in C₂+ anddenitrogenated.

The process of the invention makes it possible to separate a crude gasrich in C₂+ and in nitrogen (typically at least 1% of C₂+ and at least2% of nitrogen). According to a specific embodiment, the processaccording to the invention typically comprises the following steps:

-   -   Pretreatment of the crude gas to be treated (separation of the        water, CO₂, methanol, heavy hydrocarbons, for example).    -   Cooling of the crude gas to a first subambient temperature        (typically between −30° C. and −70° C.), making it possible to        obtain a cooled two-phase stream.    -   Separation of the cooled two-phase stream into a first gas and a        first liquid.    -   Reduction in pressure of at least a portion of said first gas in        a turbine and introducing it, after reduction in pressure, into        the middle of an upper demethanization column, known above as        first distillation column.    -   Reduction in pressure of at least a portion of the first liquid        in order to inject it into the middle of an upper        demethanization column, known above as first distillation        column.    -   Extraction of at least a portion of a gas stream at an        intermediate stage of an upper demethanization column, known        above as first distillation column, in order to inject it into a        lower demethanization column, known above as the second        distillation column, at least one theoretical stage below the        column top.    -   Obtaining, at the top of the upper demethanization column, a gas        poor in C₂+ and rich in nitrogen (richer than the crude gas).    -   Obtaining, at the top of the lower demethanization column, a gas        poor in C₂+ and poor in nitrogen (typically containing at least        two times less nitrogen than the top of the upper column and        preferably containing less than 5% of nitrogen).

This also makes it possible to considerably simplify the sheet-metalworking of the demethanization columns conventionally employed in theknown processes of the state of the art. This is because, typically, thecolumn top is much wider than the column bottom, which presentsmechanical constraints and thus additional costs. The separation of thecolumns makes it possible to escape this constraint.

In order to deplete the top of the lower demethanization column, anadditional reflux is provided. This reflux should, if possible, be verypoor in nitrogen. Several means are possible for providing this reflux:

-   -   Use of a dedicated condenser, for example with liquid methane at        a lower pressure than the column top. This liquid methane can be        produced by the downstream part of the process (NRU).    -   Use of the recompressed and recondensed top gas from the lower        demethanization column.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to theFIGURE, which illustrates a specific example of an implementation of aprocess according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the FIGURE, a flow 1 of natural gas pretreated beforehand (typicallyhaving undergone a separation of a portion of at least one of thefollowing constituents: water, CO₂, methanol, sulfur compounds, veryheavy hydrocarbons, that is to say having more than six or seven carbonatoms (such as C₈+ compounds, for example)) comprising at least 30 mol %of methane, at least 0.1 mol % of heavier hydrocarbons than methane(that is to say comprising at least two carbon atoms) and between atleast 4 mol % et 50 mol %, indeed even 80 mol %, of nitrogen isintroduced into a system 2 which makes possible an at least partialcondensation of said flow 1.

The pressure of this flow 1 is between 20 bara (bar absolute) and 100bara (typically between 30 and 70 bara) and the temperature is close toambient temperature, for example between 0° C. and 60° C.

The system 2 is, for example, a heat exchanger. The mixture 3 exitingfrom this system 2 is in a two-phase (gas and liquid) state. Thismixture 3 is introduced into a phase separator 4.

The operating pressure is between 20 and 100 bara, typically between 30and 70 bara. The temperature of this separator is between −100° C. and0° C., typically between −80° C. and −20° C.

At least a portion 8′ of the gas phase 8 resulting from the separator 4is reduced in pressure by means of a turbine 9. The flow resulting fromthe turbine 9 is introduced into a first distillation column 7 at leastone theoretical stage 10 located below the top of said column 7.

The liquid phase 5 resulting from the separator 4 is reduced in pressurethrough a valve 6 and is then injected, at a pressure of between 10 baraand 40 bara and a temperature, for example, of between −110° C. and −30°C., into said demethanization column 7. This liquid phase 5 isintroduced at a theoretical stage 10′ below the top of said column 7 andbelow the stage 10 for introduction of the gas flow 9.

At the top 14 of the first distillation column 7 (top=highest outlet ofthe column), a gas flow 15 enriched in methane, typically containingless than 0.5 mol % of hydrocarbons having more than two carbon atoms(containing at most half the amount of heavy hydrocarbons—having morethan 2 carbon atoms—present in the feed gas), is extracted. Thetemperature of the gas stream 15 is less than −80° C.

At an intermediate stage 38 (that is to say at least one stage belowthat of the top 14 of the column 7) of the first distillation column 7,a gas stream 39 is extracted in order to be introduced into ademethanization column 7′, also known below as second distillationcolumn 7′, at a stage 10′ located below that of the top 14′ of thecolumn 7′.

This gas stream 39 resulting from the first distillation column 7 isdepleted in nitrogen (typically containing less than 10%, preferablyless than 5%), just like the liquid phase 5 resulting from the separator4. Gas depleted in nitrogen is understood to mean a gas stream having anitrogen content which is less than half the nitrogen content of theinitial gas stream 1 to be treated and preferably less than a quarter ofthis content. The result of this is that very little nitrogen isintroduced into the second distillation column 7′. Consequently, the gasstream which will be extracted from this second distillation column willnot have to be introduced into an NRU unit, which will greatly lightenthe burden on this NRU unit which will have to treat the gas stream 15resulting from the first distillation column 7. Typically, between 10%and 20% of the methane initially present in the gas stream 1 to betreated will be re-encountered in this liquid stream 39 introduced intothe second distillation column 7′ and thus will not have to beintroduced into an NRU unit.

The liquid stream 39′ produced in the bottom 14″ of said seconddistillation column 7′ is extracted in order to be introduced into thelower part 50 of the first distillation column 7.

A liquid flow 12 of heavier hydrocarbons than methane is recovered inthe bottom part 16 of the column 7.

A reboiler 11 is placed at a level which makes it possible to reboil thebottom liquid from the column 7 in order to reheat a portion of theliquid from said column with the aim of adjusting the maximum thresholdfor methane present in the flow 12 of heavy hydrocarbons.

At least 50 mol % (typically at least 85 mol %) of the heavyhydrocarbons present in the gas mixture 1 to be treated are recovered inthis flow 12. Preferably, at least 90% are recovered.

Preferably, the liquid flow 12 of hydrocarbons does not contain morethan 1 mol % of methane.

A heat exchanger 13 can be installed in order to reheat the bottom partof the column 7 (bottom part=below the introduction of the liquidoriginating from the separator 4). This exchanger is fed with the gasfeed stream 1. This reheating improves the balance between search formaximum yield and purity of the outlet flow from demethanization column7.

Demethanization column is understood to mean a distillation columnintended to produce at least two streams which are different incomposition starting from a feed stream to be treated according to theprocess of the present invention. The at least two streams are asfollows: one, at the column top, gaseous, depleted in hydrocarbonshaving at least two carbon atoms, that is to say comprising less thanhalf of the “heavy” hydrocarbons present in the feed gas (ethane,propane, butane, and the like), and the other, in the column bottom, inthe liquid form, depleted in methane present in the feed stream to betreated.

The molar concentrations of the different components of the streams ofthe different steps of the process as illustrated according to theFIGURE are shown in the table below. It may then be observed that thestream 39 is a liquid comprising predominately methane and, to a minorextent, ethane and propane and contains virtually no nitrogen.

° 1 39 15 12 15′ Methane 88.6% 93.5% 93.9%  1.4% 97.4% Ethane  4.8% 5.7%  0.4% 69.9%  0.4% Propane  1.3%  0.2%  0.0% 19.7%  0.0% Isobutane 0.2%  0.0%  0.0%  2.8%  0.0% n-Butane  0.3%  0.0%  0.0%  4.5%  0.0%Isopentane  0.1%  0.0%  0.0%  1.0%  0.0% n-Pentane  0.0%  0.0%  0.0% 0.6%  0.0% Helium  0.1%  0.0%  0.2%  0.0%  0.1% Nitrogen  4.5%  0.6% 5.5%  0.0%  2.1%

Demethanization unit is understood to mean any system comprising atleast one distillation column for enriching the top gas in methane anddepleting the bottom liquid in methane.

It is possible to condense a gas enriched in methane under pressure inorder to improve the performance qualities. This condensation is carriedout by virtue of a heat exchanger 17 fed both with a portion 8″ of thegas flow 8 resulting from the separator 4 and with the gas streamenriched in methane 15 resulting from the top 14 of the distillationcolumn 7. Only an exemplary embodiment of the process which is a subjectmatter of the invention is concerned here. However, according to aspecific embodiment of the invention, a third stream to be condensedmight be introduced into this exchanger. According to yet anotherembodiment of the invention, just one of the two streams described wouldneed to be condensed.

Gas enriched in methane 15 is understood to mean a gas mixturecontaining methane, nitrogen and typically less than 0.5% hydrocarbonshaving more than two carbon atoms (containing at most half the amount ofheavy hydrocarbons—having more than two carbon atoms—present in the feedgas).

The stream or streams 18 (18 a and 18 b) which has (have) been cooled inthe exchanger 17 is (are) reduced in pressure by means, for example, ofat least one valve 19 (19 a, 19 b) and is (are) then introduced into atop part (top part=above the feed 10 exiting from the turbine 9) of thecolumn 7.

The gas stream 15′ is extracted at the top 14′ of the seconddistillation column 7′ at a temperature of between −80° C. and −120° C.and at a pressure of greater than 10 bara (typically of between 15 baraand 30 bara). This gas stream 15′ is introduced into a heat exchanger17, 27 or 2 in order to be produced at the end of the process as naturalgas at a pressure (before possible subsequent compression) close to theoperating pressure of the column 7′ (typically between 10 and 30 bara)and a temperature close to ambient temperature (typically between 0° C.and 60° C.).

The reflux of the second distillation column 7′ is provided, in the sameway as for the reflux of the first distillation column 7, by theintroduction, into its upper part 41, of at least one stream (two arerepresented in the FIGS. 18c and 18d which has (have) been cooled in theexchanger 17 and reduced in pressure by means, for example, of at leastone valve (19 c, 19 d).

Alternatively, the reflux of the two distillation columns 7 and 7′ canbe provided, at least in part, by a portion of the gas stream 15′extracted at the top of the second column 7′ and then cooled in a heatexchanger before said refluxes.

These reflux steps are necessary in order to feed the two columns 7 and7′ with cold liquid poor in C₂₊.

The flow 20 which was reheated in exchanger 17 contains at most half ofthe amount of heavy hydrocarbons—having more than two carbonatoms—present in the feed gas 1.

The gas stream 20 reheated in the exchanger 17, at a temperature ofbetween −40° C. and −70° C., preferably of the order of −60° C., issubsequently partially condensed by means, for example, of a heatexchanger 21. There emerges, at the outlet of this exchanger 21, atwo-phase (gas/liquid) stream 22 (comprising from 20 mol % to 80 mol %of gas).

Alternatively, it is possible to be exempted from the preceding step,that is to say from the passage of the stream 15, extracted from the topof the demethanization column 7, into the heat exchanger 17. It is thuspossible to keep the temperature of the stream 15 below −80° C. (or evenbelow −100° C.) and to introduce said stream 15 directly into the heatexchanger 21 in order to obtain the stream 22.

The stream 22 is subsequently sent to a denitrogenation system A. In thedenitrogenation system A, the two-phase stream 22 is, after an optionalreduction in pressure in a valve or a turbine 23, introduced into aphase separator 25. The liquid phase 29 resulting from the phaseseparator 25 is, after an optional reduction in pressure in at least onevalve 42′ (in the FIGURE, two valves 42″ and 42′ are represented),reheated through the heat exchangers 27, then 21 and finally 2 in orderto rejoin the outlet stream 30 (in the FIGURE, two outlet flows 30 and30′ are represented as, by way of the two prior reductions in pressure42″ and 42′, a medium-pressure stream 30′ and a low-pressure stream 30are produced) of gas rich in methane produced at the process outlet.

Medium-pressure is understood to mean a pressure of between 13 bara and18 bara, typically 15.5 bara. Low-pressure is understood to mean apressure of between 2 bara and 7 bara, typically 5.7 bara.

The outlet flows 30, 30′ and 40 contain less than 5 mol % of nitrogen.

The gas phase 26 resulting from the separator 25 is partially condensedin a heat exchanger 27 and then reduced in pressure at the outlet ofsaid exchanger 27 by means of a turbine or of a valve 28 before beingintroduced into a distillation column 31.

The distillation column 31 is a column for stripping nitrogen, the aimof which is to separate the nitrogen from the liquid enriched in methaneat the outlet, also known as denitrogenation column. The liquid enrichedin methane comprises less than 5 mol % of nitrogen. In this instance, adistillation column joined to a reboiler 32 but not having available anassociated condenser system is concerned.

A stream 33 very rich in methane in the liquid form is extracted at thebottom of the column 31, at a temperature of less than −100° C.,preferably of less than −110° C. This stream 33 contains less than 5 mol% of nitrogen, preferably less than 4%. The liquid stream 33 issubsequently mixed with the liquid phase 29 resulting from the phaseseparator 25 and follows the same path as far as the outlet flows 30,30′.

A gas flow 36 rich in nitrogen, at a temperature of less than −110° C.,is produced at the top 35 of the column 31. Said flow 36 rich innitrogen comprises at least 20 mol % of nitrogen.

The flow rich in nitrogen 36 is reheated through the successiveexchangers 27, 21, then 2. It can also be one and the same exchanger,according to a specific embodiment of the invention. Moreover, accordingto another specific embodiment of the invention, more than threeexchangers can be employed.

This then results in a stream 37, at a temperature close to ambienttemperature (greater than −10° C. typically and less than 50° C.), sentto an additional denitrogenation system B. The aim of thedenitrogenation system B is to produce a gas stream even richer innitrogen than the stream 37. This system B can, for example, include atleast one separator and one denitrogenation column. If the specificationfor nitrogen at the outlet of the system B is strict (<100 ppmtypically), it may prove to be necessary to add, to the system B, acycle compressor, for example a nitrogen or methane compressor, in orderto contribute the reflux necessary in order to obtain the nitrogenpurity at the top of the denitrogenation column of the system B.

A specific NRU unit has been described in this FIGURE but the processwhich is a subject matter of the present invention applies to any typeof NRU unit downstream of an “NGL” unit.

The process which is a subject matter of the present invention makes itpossible to achieve savings in terms of electricity consumption, forexample. This is because only a portion of the methane included in thegas to be treated is sent to an NRU unit, as the other portion whichoccurs in the bottom of the first distillation unit in the liquid formdoes not contain nitrogen, with the result that the NRU unit downstreamof the NGL unit is much less burdened.

Furthermore, if the FIGURE illustrating an embodiment of the presentinvention is taken up, it is found that the outlet stream 40 is alreadyat high pressure, with the result that the final user does not need touse a compressor to increase the pressure of the stream 40 (or possiblyhas a very limited compression need); only the streams 30 and 30′ willrequire significant compression: this represents a saving in electricityconsumption of the order of 10% to 30%.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1.-8. (canceled)
 9. A process for the separation of the components of agas mixture to be treated comprising methane, nitrogen and at least onehydrocarbon having at least two carbon atoms, or a mixture of thesehydrocarbons, comprising the following steps: a) introducing the gasmixture to be treated into a first distillation column therebyproducing, at the column top, a first gas stream enriched in methaneand, at a level lower than that of the top of said column, another gasstream; b) introducing said other gas stream resulting from step a) intoa second distillation column, at a level lower than that of that of thetop of said second column, thereby producing, at the top of this column,a second gas stream rich in methane, wherein the nitrogen content ofsaid second gas stream is at least 1.5 times lower than the nitrogencontent of the first gas stream and in that from 5 mol % to 30 mol % ofthe methane initially present in the gas mixture to be treated iscomprised in the second gas stream.
 10. The process as claimed in claim9, further comprising: c) introducing said first gas stream enriched inmethane resulting from step a) into a denitrogenation unit in order toseparate the nitrogen.
 11. The process as claimed in claim 10, whereinthe second gas stream resulting from step b) is not treated by thedenitrogenation unit.
 12. The process as claimed in claim 9, furthercomprising, prior to step a), the following steps: at least partiallycondensing said gas mixture to be treated in order to obtain a two-phasemixture; injecting the liquid phase from said two-phase mixture intosaid first demethanization column at a first injection stage; injectingthe vapor phase from said two-phase mixture into said firstdemethanization column at an injection stage different from said firststage.
 13. The process as claimed in claim 9, wherein the gas stream,extracted from the first distillation column in step a), comprises atmost half of the amount of hydrocarbons having more than two carbonatoms present in the feed gas.
 14. The process as claimed in claim 9,wherein said gas mixture to be treated comprises 70 mol % of methane, atleast 4 mol % of nitrogen and 2 mol % of hydrocarbons having at leasttwo carbon atoms.
 15. The process as claimed in claim 9, wherein saidgas stream resulting from step b) is extracted directly from said seconddistillation column at a pressure of greater than 20 bara and comprises95 mol % of methane.
 16. The process as claimed in claim 9, wherein aportion of the second gas stream rich in methane and depleted innitrogen resulting from step b) at the outlet of the top of the seconddistillation column is compressed and then condensed in order to beintroduced, for one portion, into the upper part of the firstdistillation column and, for the other portion, into the upper part ofthe second distillation column in order to carry out the reflux of saiddistillation columns.